The proposed studies are directed at the definition of critical surface molecules, factors, and cell interactions involved in thymocyte maturation. The intention is to investigate in detail the involvement of specific thymocyte surface molecules in human thymocyte activation and differentiation. For this purpose an in vitro thymocyte culture system, which has been developed and characterized by the applicant, will be used. Monoclonal antibodies to T cell antigens will be used to isolate relevant thymic subpopulations as well as to probe the role of the selected thymocyte surface antigens in the mediation or regulation of discrete stages of thymocyte maturation. Monoclonal antibodies to T cell surface antigens, MHC antigens, and specific lymphokines will be utilized for a detailed investigation of interactions between isolated subpopulations of immature thymocytes and non-T cells. These studies will take advantage of the availability of a large panel of monoclonal antibodies and the development of fluorescence flow cytometric techniques (including two color fluorescence) for the isolation of thymic subpopulations and the analysis of thymic surface antigen expression. These studies will take advantage of the availability of recombinant or purified lymphokines which may be critical in thymocyte development and growth. In addition to defining critical thymocyte surface molecules and factors, it is hoped that these studies may lead to an understanding of regulatory events and MHC restrictions which occur during thymic ontogeny. The information gained may contribute greatly to our understanding of normal human T cell differentiation and abnormal T cell differentiation which occurs in various T cell malignancies, autoimmune and immunodeficiency diseases.